The present invention relates to solid state image pick-up devices, in general, and to apparatus for improving the resolution and/or color balance of an image sensed by a photosensor array through a color optical filter in an electronic camera, in particular.
It is well-known to employ tricolor striped or mosaic-type optical filters in combination with a plurality of photosensors such as an array of charge coupled devices (CCDs) in an electronic camera for color image sensing purposes. The color component of the scene image is established by the optical filter and the CCD stores an electrical charge in proportion to the level of filtered scene light incident thereon. A tricolor optical filter of the type normally combined with a CCD array for color image sensing purposes might comprise the primary colors of red, green and blue in an additive process, or the primary colors of yellow, green and cyan in a subtractive process with the colors being regularly arranged on the CCD array in either stripe or mosaic form. Other additive and subtractive primary color combinations are also employed in a colored optical filter for such CCD array scene light filtering purposes.
A combination tricolor optical filter and CCD array generate electrical signals representative of a particular scene when an image of the scene is focused on said CCD array through said optical filter. In order to generate an electrical signal representative of any particular color present in a scene and imaged on a CCD array a group of three CCDs and their associated color filters that collectively include three primary colors must be employed. The term "primary colors" as employed herein means a set of colors from which all of the colors may be derived. As a consequence of this requirement, the ability of a combination tricolor filter and CCD array to resolve or distinguish between scene objects separated by small angular distances is limited by the area encompassed by the smallest three CCD combination in any given size CCD array.
If, for example, a tall and thin scene object such as a blue flag pole was imaged on a three CCD combination such as that mentioned above where each CCD in the combination included either a yellow, green or cyan subtractive filter, the CCD having its incident light filtered by a yellow filter would not "see" the blue flag pole because a yellow subtractive filter absorbs blue light and therefore does not allow blue light to reach its associated CCD element. The same would be true of the green filter and its associated CCD element. The only blue light to reach a CCD element through either a yellow, green or cyan filter of the three CCD combination mentioned above would be the blue light reaching the CCD associated with the cyan filter. Therefore, in this particular example two-thirds of the light sensing area associated with this three CCD combination and therefore two-thirds of the light being imaged on said three CCD combination cannot be employed to resolve an image of the above-mentioned tall and thin blue flag pole.
One obvious solution to this image resolution problem would be to increase CCD density or the number of CCDs per unit area within the same area covered by a lower density CCD array. While this approach may be effective in resolving most image resolution problems, a significant cost penalty would be incurred for the higher density CCD array which may render the device in which it is employed economically impractical.
In addition to the above-mentioned image resolution problem associated with the absorptive color filtering of scene light subsequently imaged on a CCD array, present CCD light filtering arrangements employ color filters that transmit a particular scene color more readily than other scene colors, thereby creating an imbalance or overly dominant color in an electronic image subsequently generated by such a CCD array. If, for example, one-third of the CCDs in a CCD array were filtered by either a subtractive yellow, green or cyan filter, green light would be transmitted to every CCD in the array, whereas red and blue light would be transmitted by only one-third of the CCDs in such an array.
One arrangement reported to have been employed by others to correct the above-noted color imbalance is the technique of moving a dominant color absorbing subtractive filter into and out of the path of the image-containing light rays impinging on an optically filtered CCD array during each CCD array exposure interval. An exposure interval is defined herein as a fixed period of time during which it is assumed that there will be no change in the scene to be imaged on a CCD array through a subtractive color filter. In one portion of an exposure interval (consisting of two separate exposures) all of the scene colors impinge on the tricolor optical filter. In another portion of the same exposure interval, the subtractive filter is inserted into the path of the image-containing light rays impinging on the optical filter which thereby blocks the dominant scene color and transmits less dominant scene colors to the tricolor optical filter. In this manner, the level of the less dominant scene colors sensed by the CCD are increased relative to the dominant scene color which results in an electronic image that is substantially more color balanced than one where a dominant color subtractive filter is not employed for such color balancing purposes. While this arrangement is effective for improving the color balance of a reconstructed image, it has no effect on the image resolving capabilities of the CCD array.
Therefore, it is a primary object of the present invention to improve the image resolving capabilities of a photosensor array that senses scene images through a colored optical filter.
It is another object of the present invention to improve the image resolving capabilities of a photosensor array that senses scene images through a colored optical filter without increasing the density of the photosensors forming said photosensor array.
It is a further object of the present invention to improve the image resolving capabilities of a photosensor array that senses scene images through a colored optical filter while at the same time improving the color balance of an electronic image subsequently generated by such an array.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiment thereof taken in conjunction with the accompanying drawings.